The invention relates to a method for uniformly heating, particularly for the purpose of preserving organic products or substances (referred to herein simply as "products" for the sake of brevity) in a container which can be penetrated by electromagnetic waves. The invention is intended to serve particularly for the heat treatment of preferably water-and/or albumin-containing foodstuffs and products intended for human consumption, such as for example dairy products, meat and internal organs.
Perishable products, especially organic substances, such as for example foodstuffs, are preserved, inter alia, by heat treatment. In order to avoid denaturing of the product, the heating must be preformed as carefully as possible. "Denaturing" is to be here understood as the loss of desirable ones of the original characteristics of the product such as cannot be restored, for example the loss of water-solubility of albumin as a result of coagulation in meat, in milk or in poultry.
With conventional processes (cooking, baking, autoclaving, grilling or frying) the heat energy is applied to the product in the form of contact heat or as infrared radiation. The heat is applied to the product from outside the product until the product is warmed through, i.e., until the temperature necessary for preservation is established in the innermost parts of the body or mass. The external surface and the outer parts of the body or mass are accordingly heated for a longer time and/or to a higher temperature than the more internally located layers. As a result, even with relatively small bodies or masses to be treated, it is impossible to avoid denaturing of at least parts of the outer layers, if the layers of the body or mass in the interior thereof are to be preserved.
Accordingly, contact heating and infrared radiation are suited in the main for the preservation of products, such as many meat and fish preserves, which suffer no loss of quality worth mentioning when subjected to prolonged and/or relatively intense heating.
When a product is heated by electromagnetic alternating fields the energy serving for the heating is not applied in the form of heat, but instead is converted into heat within the product itself, as a result of which there is produced a relatively uniform heating of all those parts of the product lying within the penetration depth of the field being used.
The penetration depth of the electromagnetic alternating field being used depends upon its frequency, as indicated by the following equation. ##EQU1## Z: is the penetration depth at which the energy density has decreased to 1/e its incident value; k' is the dielectric constant of the product relative to vacuum (.epsilon.'/.epsilon..sub.o); and l.sub.o is the wavelength of the radiation in free space.
Since wavelength and frequency are inversely proportional to each other, the penetration depth decreases as the frequency is selected higher and higher.
For heating perishable products use is made of alternating electromagnetic fields (referred to herein for the sake of brevity as electromagnetic fields or simply as fields) which we can divide on the basis of frequency into two groups:
1. fields with ultra short waves and decametric waves, and PA1 2. fields with centimeter waves and decimetric waves.
Both of these groups have characteristics which are of significance when these fields are used for the heat treatment of perishable products.
The relatively low-frequency fields of the first ultra short and decametric wave group (referred to hereafter as "first electromagnetic alternating fields" or for the sake of brevity as "first fields"), as can be derived from the foregoing equation, have a relatively large penetration depth which by appropriate selection of frequency can be readily adjusted for the process to be used, for the particular product to be treated, and for the product thickness. Generation of these fields requires only relatively little expense.
However, industrial use of fields with decametric and ultra short waves for the preserving of products results in practical problems: If large potential differences are established discharges frequently occur, then these can damage the product or render it useless. These potential differences result, inter alia, from the high field intensities necessary for the heating. Such discharges frequently occur at or near the boundary layers between the product to be treated and the surrounding air or other gas.
Special problems result from the use of fields of this frequency group for heating or preserving when there is gas present above the product in the container in which the product is located. In such cases, the discharges occur not only outside the container or wrapping but also within such gas, inside the container, resulting in perforation of the container walls.
With the fields of the second group there is no danger of such occurrences. On the other hand, these centimeter and decimetric waves, as can be derived from the foregoing equation, penetrate into only a relatively shallow layer located near the external surface of the body or mass to be treated. For example, the frequently employed frequency of 2.450 MHz results, in the case of red, lean meat, in a useful heating action to a depth of only about 20 millimeters, which seldom suffices for the preservation of foodstuffs. Additionally, relatively high expense is involved in the generation of fields of these frequencies.